Equations Friday, 16 November 2018 L.O: Know that word equations and symbol equations can be used to represent reactions, and to be able to write word equations. To be able to calculate the mass of a product or a reactant from the masses of other substances involved in the reaction and be able to write balanced symbol equations. (Higher)

H K B Pb Na Sc Kr Cs Zn Starter What is the element that each of these symbolise? Click on the boxes to reveal the answer… Pb H Na Lead Sc Hydrogen K Sodium Scandium B Potassium Kr Cs Zn Boron Krypton Caesium Zinc

Word and Symbol Equations Word equations and symbol equations show what happens in a chemical reaction. They show the reactants (the substances that react together) and the products (the substances that are made in a reaction). EXAMPLE The reactants were Magnesium (2Mg) and Oxygen (O2) The product was Magnesium Oxide (2MgO) EXAMPLE Here is the balanced symbol equation: Now copy this into your book, and answer the questions using the symbols: What are the reactants? What is the product? EXAMPLE Magnesium and oxygen react to form magnesium oxide. This can be represented by a word equation or a symbol equation. EXAMPLE Here is the word equation: Copy it into your book, and answer the questions: What are the reactants? What is the product?

Conservation of Mass REMEMBER: During chemical reactions, things don't appear out of nowhere and things don't just disappear. You still have the same atoms at the end of a chemical reaction as you had at the start. They're just arranged in different ways. Balanced symbol equations show the atoms at the start (the reactant atoms) and the atoms at the end (the product atoms) and how they're arranged.

Conservation of Mass Because atoms aren't gained or lost, the mass of the reactants equals the mass of the products. This is called conservation of mass. You can use this fact to work out the mass of individual reactants and products in a reaction. EXAMPLE QUESTION 30 g of magnesium oxide is formed from 18 g of magnesium. How much oxygen reacted? ANSWER The total mass of the product is 30 g, so the total mass of the reactants must be 30 g. The mass of the magnesium is 18 g, so the mass of the oxygen must be30 – 18 = 12 g. EXAMPLE QUESTION 6 g of magnesium completely reacts with 4 g of oxygen.  What mass of magnesium oxide is formed? ANSWER The total mass of the reactants is 4 + 6 = 10 g, so the mass of the product (magnesium oxide) must be 10 g.

Balanced Equations There must always be the same number of atoms of each element on both sides of an equation — they can't just disappear. If there aren't the same number on each side then the equation isn't balanced.

Method for balancing equations You can balance equations by putting numbers in front of the formulas where needed. All you do is this: Find an element that doesn't balance and pencil in a number to try and sort it out. See where it gets you. It may create another imbalance — if so, just pencil in another number and see where that gets you. Carry on chasing unbalanced elements and it'll sort itself out pretty quickly.

EXAMPLE – Balancing equations H EXAMPLE – Balancing equations But now you have too many H atoms and O atoms on the right-hand side, so to balance that up you could try putting 2NaOH on the left-hand side. In this equation we're short of H atoms on the right-hand side — there are three H atoms on the left and only two on the right. The only thing you can do about that is make it 2H2O instead of just H2O: And suddenly there it is! Everything balances. There are four H, one S, six O and two Na on each side of the equation. H = 4 S = 1 O = 6 Na = 2 H = 3 H = 4 S = 1 O = 5 O = 6 Na = 1 Na = 2 H = 3 H = 2 S = 1 O = 5 Na = 1 Na = 2 NEXT NEXT

EXAMPLE – Balancing Equations In this equation we're short of Cl atoms on the left-hand side. What would you change it to? A: Try making it 3Cl2 instead of just Cl2. What is the problem now and how would you deal with it? A: That causes too many Cl atoms on the left-hand side, so balance up the Cls by putting 2 before the AlCl3. How would you now finish it off? A: You can balance the Al atoms by adding a 2 in front of the Al. Now everything is balanced! There are 2 Al atoms on each side and 6 Cl atoms on each side. ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?

Balanced Symbol Equations Symbol equations show how many atoms of one element there are compared to the number of atoms of other elements. So it's fine to double, or triple, or quadruple the number of atoms in a balanced equation, as long as you do the same to every term in the equation. THE NUMBERS ADDED TO THE EQUATION TO BALANCE IT HAVE ALL BEEN DOUBLED. BALANCED SYMBOL EQUATION TO SHOW THE REACTION OF ALUMINIUM AND CHLORINE THE EQUATION IS STILL BALANCED.

Answers a) Iron sulfate and copper b) Copper sulfate and iron c) copper sulfate + iron  iron sulfate + copper a) copper + oxygen  copper oxide b) 127 + 32 = 159 g. No atoms are lost or made during a chemical reaction, so the total mass at the end of a reaction is the same as the total mass at the start of the reaction. 68 – 56 = 12 g. The total mass of nitrogen and hydrogen must equal the mass of ammonia produced. a) sodium hydroxide + hydrochloric acid  sodium chloride + water b) Total mass of reactants = 80 + 73 = 153 g. 153 – 36 = 117g So 117 g of sodium chloride are formed.

Answers a) b) c) d)